The invention relates generally to the cleaning of metal surfaces. More specifically, the invention pertains to a composition useful for the dynamic cleaning of such metals as Invar alloy, which is a nickel-iron alloy especially suited for cryogenic temperatures because of its low thermal expansivity. A representative sample of this alloy contains the following elements by weight percent:
Iron 61.4 - 64.4 Nickel 34.5 - 36.5 Cobalt 0.50 Titanium 0.30 -0.60 Manganese 0.30 -0.60 Silicon .ltoreq.0.30 Carbon .ltoreq.0.05 Phosphorus .ltoreq.0.015 Sulfur .ltoreq.0.015 Lead .ltoreq.0.015 Tin .ltoreq.0.015 Selenium .ltoreq.0.010
Cleaning alloys for use in cryogenic applications, such as piping for extremely low temperature fuel and oxidizer systems in rockets and spacecraft presents difficult problems: for example, the systems must be devoid of all hydrocarbon and particulate contamination. Conventional cleaning solvents are not satisfactory for the present purpose.
Although acids such as phosphoric, sulfuric, and hydrochloric display low corrosion rates, they produce objectionable smut on the metal surface. Smut is defined as the accumulation of noticeable amounts of nonadsorbent products on chemically treated metal surfaces, usually resulting from cleaning or etching. Use of oxidizing agents in conjunction with acids increases corrosion rates without lessening the smut formation. Although the nickel-iron alloys have good corrosion resistance to caustic solutions at ambient temperature, effective cleaning and degreasing with alkaline solutions requires elevated temperatures, where corrosion rates become objectionable.
Consequently there is a need for a cleaning solvent which satisfactorily descales, cleans, and degreases metal surfaces, particularly iron-nickel surfaces, without leaving smut or causing excessive corrosion.